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Learning how to prevent or stop plant diseases in your garden can save you a lot of time and heartache. Use these helpful tips to enjoy a healthy organic garden! 


It’s hard to believe that we’re already heading towards the tail-end of summer! Even though we’re still harvesting baskets full of summer produce from our garden, we’ve just started many of our fall seeds indoors to get a jump start on the cool weather gardening season.

Cool weather seedlings growing under our DIY grow light system. These will be transplanted outdoors in late summer for the fall garden.

Cool weather seedlings growing under our DIY grow light system. These will be transplanted outdoors in late summer for our fall & winter garden.

This is also the time of year that your summer plants might start showing signs of bacterial or fungal disease. For instance, your tomatoes might have septoria leaf spot. Or maybe your squash have powdery mildew on their leaves.

The good news is that there are steps you can take to easily prevent and/or stop plant diseases in your summer garden. Doing so will help you increase your yields.

Some of our recommendations will also help you improve the biological fertility of your soil, which means healthier plants and larger yields from your upcoming fall and winter garden as well!

1: Focus On Soil Health (Prevention)

Biological systems and lifeforms are complex beyond comprehension. As such, it’s much easier to practice prevention than treatment. The same applies to people. For instance, it’s much easier NOT to eat a diet of highly processed foods than it is to try to treat heart disease.

Likewise, it’s important for you to understand that the foundation of any organic garden or farm is the health of its soil. Without healthy soil, trying to grow plants will be a never-ending battle of treating symptoms.

This is a soil profile from Tyrant Farms showing a young Jerusalem Artichoke growing in a no-till organic garden bed. The soil started off as brick hard, red clay. After top-dressing the bed with leaf and wood chip mulch (rather than tilling), you can see clearly how soil organic matter and soil structure has been drastically improved in a short period of time by trillions of macro and microorganisms.

This is a soil profile from Tyrant Farms showing a young Jerusalem Artichoke growing in a no-till organic garden bed. The soil started off as brick hard, red clay. After top-dressing the bed with leaf and wood chip mulch (rather than tilling), you can see clearly how soil organic matter and soil structure has been drastically improved in a short period of time by trillions of macro and microorganisms.

When we say “healthy soil” we simply mean soil that has all the biological lifeforms present to:

a) feed your plants all the macro and micronutrients they need while optimally cycling nutrients and water, and

b) protect your plants from pathogenic/disease-causing organisms.

If you’d like to learn more about how to build the biological fertility of your soil, read our article Preparing your garden beds for spring.

2: Stop Powdery Mildew on Squash, Pumpkins, Cukes, and Other Cucurbits

What a healthy summer squash plant should look like! “Powdery mildew” is a fungal disease that looks like white splotches of powder on the leaves of your cucurbit leaves, which are prone to form during hot humid/wet weather.

What a healthy summer squash plant should look like! “Powdery mildew” is a fungal disease that looks like white splotches of powder on the leaves of your cucurbit leaves, which are prone to form during hot humid/wet weather.

What is powdery mildew?

Powdery mildew” is the common name given to any number of fast-growing, airborne fungal diseases that cause white powdery patches on the leaves of cucurbits and other garden plants. Left untreated, powdery mildew causes the leaves to turn brown and dry out, eventually killing the plant.

Break out the synthetic fungicides? No! Not unless you want to also kill the beneficial fungi in your garden that help your plants.

Powdery mildew can be easily prevented using organic methods. The easiest, most proven method of treatment involves milk. Yes, milk. That white liquid from cow utters. 

How to stop powdery mildew:

Make a mixture of milk and water (30% milk to 70% water is fine) and spray it evenly on the surface of the leaves of affected plants during the morning on a sunny day.

Any type of milk will work: skim or whole. In studies, this method has proven to be as effective as any synthetic fungicide in stopping powdery mildew, although scientists aren’t quite sure how it works. (It’s likely an antiseptic effect resulting from the sun burning the fungus as it’s bound by the milk protein.)

3: Prevent or Stop Tomato Foliar Diseases

If you live in the hot humid south like we do, it’s extremely unlikely that you’ll get through a whole summer without seeing some type of foliar (leaf) disease on your tomato plants. This is especially true if you’ve been growing tomatoes for more than a year without “crop rotation.”

Crop rotation is planting different crop plant species in the same plot in subsequent years to prevent the buildup of pests and pathogens that prefer certain types of plants. The years between replanting and the types of cropping systems used can vary.   

There are probably hundreds if not thousands of bacterial, fungal, or viral foliar diseases that LOVE tomatoes, so we’re not going to bother to get too specific here.

We recommend fighting biology with biology. This is the start of a hot compost pile using the Berkeley Composting Method (developed by Berkeley University). The pile is turned every 48 hours to prevent it from going aerobic or getting too hot. The internal temperatures hit 140+ degrees F for over 10 days, pathogenic microorganisms and seeds are

We recommend fighting biology with biology. This is the start of a hot compost pile using the Berkeley Composting Method (developed by Berkeley University). The pile is turned every 48 hours to prevent it from going aerobic or getting too hot. The internal temperatures hit 140+ degrees F for over 10 days, pathogenic microorganisms and seeds are “burned” out, leaving only good biology for you to work with. Using this method, you can go from start to finished compost in as little as 3 weeks. The compost can be applied to your soil surface or made into an aerated “tea” to be used as a soil drench of foliar spray.

What’s the best organic way to prevent or stop tomato foliar diseases? For prevention and treatment, we recommend either:

a) Actively Aerated Compost Tea (AACT) 

We recommend making AACT from hot compost (Berkeley Method) or high quality worm castings.

How To Use It 

Apply AACT as a foliar spray throughout the growing season. Also use it as a soil drench 1-2 times per year during seasonal transitions (summer > fall garden or winter > spring garden). During particularly rainy periods when disease pressure will be higher, you might want to increase application frequency.

How It Works 

The best analogy here is the human microbiome, e.g. the trillions of microorganisms in and on you that play a wide range of roles in keeping you healthy and alive. If your gut flora is out of balance, it can have profound impacts on your physical and even psychological health.

The same is true with plants. By keeping their leaf surfaces coated with beneficial bacteria, fungi, etc, you make it exponentially more difficult for pathogenic microorganisms to take hold or proliferate. Similarly, you can keep soil-borne pathogens in check by applying AACT as a soil drench.

b) Serenade 

If you don’t feel like making Berkeley hot compost (it’s a lot of hard work) or making worm casting tea, you can take the easier, store-bought approach. In that case, we recommend using Serenade (you can buy it here).

Serenade is OMRI listed for organic farms and gardens.

How To Use It 

Use Serenade preventatively as a foliar spray or immediately upon seeing signs of disease on your tomato plants’ leaves. You can buy it as a pre-diluted, ready-to-use spray bottle or in concentrate to mix at home as-needed.

How It Works 

Serenade is a patented strain of bacteria (Bacillus subtilis) that consumes many types of pathogenic bacteria and fungi or outcompetes them for prime real estate on the surface of your plants. Actinovate is another excellent bio-based organic product that is also good at preventing or stopping plant diseases.

4: Proper Irrigation Methods

We recommend using drip irrigation rather than overhead irrigation or sprinklers if at all possible.
Drip irrigation

Irrigating your plants with a sprinkler/overhead irrigation is less water efficient than irrigating through drip lines (overhead sprinklers can lead to water losses up to 50% due to evaporation). Also, sprinklers cause your plants’ leaves to get wet and therefore makes them more prone to infection by pathogenic fungi and bacteria (foliar diseases). If you absolutely have to use sprinklers, irrigate in the early morning to maximize water efficiency and give your plants’ leaves plenty of time to dry off!

5: Don’t Touch and Spread!

Want to touch and cuddle your plants? We’re gardeners too so we know how you feel! But if your plants’ leaves are either: a) wet, or b) showing signs of disease (discoloration or spotting) outside of normal seasonal color changes as the weather cools, you need to learn to control your plant-touching urges. Otherwise, you might well end up spreading a foliar disease to other plants in your garden.

concord grapes growing - prevent plant disease

Concord grapes at Tyrant Farms. Always clean your clippers between trimming different plants to prevent the spread of disease!

On a similar note, be sure to practice good “clipper hygiene” by washing or disinfecting your garden tools between uses–especially if you’ve used those tools to remove diseased plants or plant foliage. Rubbing alcohol or scalding hot soapy water should do the trick.

You wouldn’t want to see a doctor performing surgery on a patient without first disinfecting their hands and surgical tools, would you? The same principle applies to your garden plants. Be a good plant doctor!

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